Generally speaking, a shock absorber comprises a cylinder having a rod slidable therein by means of a piston which defines at least two, and preferably three, chambers.
Communication means are provided in the piston to enable fluid to pass from one chamber to another via fluid passages of predermined cross section.
By way of example, the accompanying FIG. 1 shows a conventional shock absorber.
It comprises a hollow cylinder 1 in which a hollow rod 2 is slidably mounted.
The rod 2 is slidable in the cylinder 1 by means of a piston 3 which divides the interior 4 of the cylinder 1 into two separate chambers 5 and 6 whose volumes vary inversely as the piston 3 moves along the cylinder 1.
Further, in the illustrated example, the piston 3 includes two communication means of the non-return type.
A first non-return valve 7 serves to put the chamber 5 into communication with the chamber 6, and is placed to allow free fluid flow from the chamber 5 into the chamber 6, while providing considerable throttling when the fluid is made to flow from the chamber 6 into the chamber 5.
A second non-return valve 8 serves to put the chamber 5 into communication with the chamber 9 inside the rod 2, and is placed to pass fluid preferentially from the chamber 9 to the chamber 5, while providing considerable throttling, i.e. restriction to the flow of fluid, from the chamber 5 to the chamber 9.
As is well known to the person skilled in the art, the rod 2 enters the cylinder 1 via a sealed passage 10 having a substantially leak-proof sealing ring 11. Similarly, the piston 3 is completely surrounded by a sealing ring 13 which slides against the wall 12 of the cylinder 1 and provides moderately good sealing, even though it need not be completely leak-proof.
Although the operation of such a shock absorber is known, it is recalled that when such a shock absorber is used to connect the chassis of a vehicle to a running gear such as a wheel, and when the wheel undergoes shock, the shock causes the rod 2 to move relative to the cylinder 1.
There are two possible cases:
In the first case the rod 2 is pushed further into the cylinder causing the fluid in the chamber 5 to pass relatively easily to fill the chamber 6, while being throttled by the valve 8 as it flows into the chamber 9.
It should be observed that the fluid is generally an incompressible oil which does not completely fill all three of the chambers 5, 6, and 9, but which leaves an empty space above a fluid top surface, i.e. in the present example, above the surface 14 of the fluid contained in the rod 2 and below the end 15 of the rod. The empty space is filled with a fluid such as air or some other compressible gas, thus providing a fluid spring to return the rod to its equilibrium position once a shock has been absorbed.
The second case is the opposite of the first, i.e. the rod 2 is pulled further out from the cylinder 1. In this case the fluid contained in the chamber 9 passes readily into the chamber 5 without significant throttling. At the same time the volume of the chamber 6 tends to be reduced, thereby driving fluid from the chamber 6 into the chamber 5. This fluid is throttled by the valve 7 thus providing the desired damping just as the valve 8 provided the damping in the first case.
Such shock absorbers are well known to the person skilled in the art. They have been in use for a long time, in particular for equipping numerous vehicles such as those mentioned above.
Of such vehicles, particular mention can be made of aircraft of all types, e.g. airplanes and helicopters. Known shock absorbers provide good results and have been successful up to the present. However, persons skilled in the art have understandably sought to further improve this type of shock absorber which, under some conditions, suffers from a drawback in that the valves 7 and 8 throttle at a rate which is structurally determined and which is therefore set once and for all during manufacture. In other words it is impossible to vary the degree of fluid damping provided by the throttling, and this can be inconvenient for shock absorbers mounted on aircraft.
Such vehicles do not always encounter the same running conditions on the surface of all the different runways they use. Surface smoothness is not the same from one runway to the next, and some have bumps in unexpected places.
Thus, to improve such shock absorbers, the Applicant has studied a particular type of shock absorber in which the values of the fluid passages through different valves such as the valves 7 and 8 are capable of being adjusted.
One embodiment of such a device is described, in particular, in French patent application No. 80/18733 filed on the Aug. 29, 1980 and entitled "Shock Absorber".
In said device, the sections of the various valve passages are controlled by sliding rods which enable the values of said fluid passage cross sections to be varied, e.g. as a function of the forces which the shock absorber has to absorb or damp.
Said device has proved satisfactory, but the Applicant has sought to obtain even better results, in particular by enabling the valves which control the fluid passages to be moved more quickly and more easily.